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Author Topic: REQ: What if you feed 12 VDC into an 12 VAC unit ?  (Read 1762 times)
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frasenci
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« on: February 10, 2010, 04:52:56 16:52 »

Can anybody answer about this please ?
I have an DSL Router wich is connected by means of an AC to AC adapter. 220 VAC to 12 VAC.
The adaptor is boken.
Question is, what happens if I replace the broken AC to AC adapter with a more common AC to 12 VDC one ?
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Francisco

2010/02/10
Thank you all boys and girls. The pletora of answers proves how alive this community is.
You all have contributed a bit of knowledge to this electronically-inclined Mechanic !
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« Last Edit: February 10, 2010, 11:00:48 23:00 by frasenci » Logged
oldvan
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« Reply #1 on: February 10, 2010, 05:03:01 17:03 »

Unlikely to work, could do damage.

The usual reason for feeding AC to the device is to be able to produce both positive and negative supplies inside the device.
Feeding it DC won't allow this.

I'd suggest finding a correct adapter.
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« Reply #2 on: February 10, 2010, 05:09:22 17:09 »

I think a DSL router do not use the ac waveform for a specific purpose, but who knows?

Just try it, as long as it is the same voltage then it won't hurt your router.

-ichan
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solutions
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« Reply #3 on: February 10, 2010, 05:51:14 17:51 »

I'd think there's a bridge rectifier at the entry point, so nothing bad should happen.  That's just a guess of most probable though.

Note that you have a one in two chance of guessing the correct DC polarity, unless you get clever with an ohmmeter first.  Wrong polarity means blocked current by the rectifier
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TomJackson69
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« Reply #4 on: February 10, 2010, 09:25:25 21:25 »

Why don't you open the DSL Router to see if there is a bridge rectifier in the input voltage. If the bridge rectifier is the first component that the AC input feed into; then you could use the 12V DC. The only thing you would want to make sure is the polarity. I suggest you draw out the rectifiers (4 rectifiers for full bridge, or at lest two). If half bridge then find the leg that connect to GROUND, that is the DC GND. If there is a full bridge, then the GROUND is more likely not connected to the AC input. In that case, I suggest open the AC to DC adapter, remove the bridge rectifiers and the capacitor; this way, you have an AC adapter.
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oldvan
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« Reply #5 on: February 10, 2010, 10:23:19 22:23 »

...I suggest open the AC to DC adapter, remove the bridge rectifiers and the capacitor; this way, you have an AC adapter.

Only works if transformer-type adapter;  a switchmode one will not allow this.
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carbontracks
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« Reply #6 on: February 11, 2010, 02:53:21 02:53 »

If there's just a full bridge rectifier on the input, then DC of either polarity should work.

Also keep in mind that a AC adapter that says 12VAC is referring to 12V rms, to it actually will have an amplitude of around 17.1V.  A 12VDC supply might not be enough for the circuitry inside.
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« Reply #7 on: February 11, 2010, 07:14:01 07:14 »

i dont know about router, but for modem it wont work, it needs ac volt for proper working
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« Reply #8 on: February 11, 2010, 10:26:11 10:26 »

Be careful before trying this. If there is a transformer at the input it will appear as a short circuit to the dc PSU. Do as suggested and open it up and look. I don't expect there will be a transformer but better safe than sorry.
Good luck
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TomJackson69
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« Reply #9 on: February 12, 2010, 05:18:49 17:18 »

Only works if transformer-type adapter;  a switchmode one will not allow this.

Hi Oldvan,
Yes, you are correct. I forget to mention about what type of wall-plug power supply.

Tom
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« Reply #10 on: February 12, 2010, 06:06:02 18:06 »

I think the router has no need for AC supply, but inside it converts the AC to DC. So it converts the 12VAC to ~17VDC. I think there'll be no problem if you use 17VDC. I think it might not function properly if you give 12vdc. Maybe inside it converts the 12vac to 17vdc to 12vdc with a regulator, which will not be possible if you give 12v input. Not sure, but might work with 15v also. Best would be to find a suitable 220 to 12VAC adapter / transformer, or use a 220VAC to ~17V DC adapter.
« Last Edit: February 12, 2010, 06:08:19 18:08 by tAhm1D » Logged
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